1. Field of the Invention
The invention relates to formulated reagents and methods for preventing or reducing scale formation in and/or on production equipment at the different stages of the phosphoric acid production process.
2. State of the Art
Although phosphoric acid can be prepared by three routes—the thermal process, the wet process, and the dry kiln process—the wet process is the most commonly-used process in phosphoric acid production. In the process, calcium phosphate rocks, which contain mostly calcium phosphate, are cleaned in the wash plant and grinded in the Ball mill before fed into a series of reactors for digestion with sulfuric acid along with recycled phosphoric acid from the process. The digestion temperature typically ranges from 40° C. to 80° C. After completing the reaction series, the process stream is washed with evaporator condensate while being forced through a filter.
After digestion, the reaction slurry is filtered to separate phosphoric acid from Gypsum (calcium sulfate). The filtered, crude phosphoric acid is then sent to Clarifiers and Evaporators for further purification and concentration. The purified phosphoric acid is either sent out as Merchant Grade Acid (MGA) or continued to make 69% P2O5Super Phosphoric Acid (SPA). The Gypsum is washed and dried before being sold for commercial uses. Some of the crude phosphoric acid is concentrated to 44% (P2O5) before sent for Monoammonium Phosphate (MAP), Diammonium Phosphate (DAP) and ammonium phosphate-sulfate (APS) production.
Due to the supersaturated nature of the acid and the impurities in the phosphate ores, the concentration steps with respect to P2O5 render several side reactions, causing scale formation in and/or on the equipment at different stages of the phosphoric acid production. For example, fluorosilicate is one of the more common scale species found in phosphoric acid production. It can be depicted by the following equations:

More than 12-15 other types of scaling species can be found throughout the phosphoric acid production process and they pose significant challenges for the industry. Plants normally have to shut down production every few weeks to physically clean up the scale using high pressure water and/or mechanical means. The economic impact for the scale-related issues is substantial, and the industry is in need of a more efficient scale prevention technology than the existing physical means of post scale formation removal.
Conceptually, there are two basic types of approaches scale removal from the phosphoric acid production process—namely, the physical method and the chemical method. There are several options for the physical method. In addition to the previously mentioned mechanical and water wash method, magnetic separation (Wang, Chuhua; Benson, Robert F.; Martin, Dean F. Enhanced solubility of sodium fluorosilicate scale by magnetic treatment, Florida Scientist (1998), 61(1), 17-25) and ultrasonic methods (Pandey, A. D.; Mallick, K. K.; Pandey, P. C.; Varma, S. Prevention of scale deposition on heat exchanger surfaces by use of high intensity ultrasonic waves during concentration of wet process phosphoric acid, Fertiliser News (1983), 28(6), 45-8) have also been used as part of the physical approach. Another approach still, is available by using physically smoothed piping in phosphoric acid production (See DE 3039187).
With regard to the chemical method, this approach is normally done by adding reagents to change the degree of supersaturation, either to induce precipitation before filtration, or to prevent scale from forming. This is the preferred approach because it requires a limited amount of capital investment and does not alter the existing process in the phosphoric acid plants. It also does not require a large amount of reagent and is therefore considered both environmental, and to have a minimal impact downstream.
However, most of the existing work addressing the scale problem in phosphoric acid production plants is based on the work for addressing scale issues in water boiler system. Some examples for scale treatment in boiler water are such as copolymers of acrylic acid and 2-acrylamido-2-methylpropane sulfonic acid (AMPS) (EP0271035). These polymers were reported to reduce the amount of silica gel adhering to the wall of the testing bottles. Other systems such as polyamine, phosphonic acid and carboxylic acid based monomers and polymers have also shown effectiveness in scale removal in boiler water system (See, e.g., GB2424876, JP2002263690, and EP0677485).
Because water boiler systems differ vastly from the wet-process phosphoric acid production environment, this system does not provide the best model for use in the phosphoric acid production process. The water boiler systems usually have mild condition with a pH in the range of 8 to 9, and a low concentration of dissolved salts. The wet-process phosphoric acid production environment, by contrast, normally contains harsh conditions with a low pH and a high solid content. Additionally, the scale in phosphoric acid plants have much more complicated components—containing more than 15 known species, such as Na2SiF6, K2SiF6, CaSiF6.2H2O, CaF2, MgF2, CaSO4.2H2O (Gypsum), MgSiF6.6H2O, Mg0.8Al1.5F6.XH2O (wherein X is a variable integer), MgH2P6O7, CaSO4, Al(PO3)3, NaK2AlF6, Ca3(AlF6)2.4H2O, MgNaA1F6.2H2O, Ca4SO4AlSiF13.10H2O (see for example, A. William Frazier, James R. Lehr, and Ewell F. Dillard, Environmental Science 8. Technology, 11, 1007, 1977). Moreover, different phosphoric acid plants experience different types of scale, and even within one plant, the type of scale can differ greatly from one location of the process to the other. Due to the complexity of the scale forming issues, it is a great challenge to develop scale inhibition reagents for use in preventing and/or reducing scale in phosphoric acid plants.
For example, in a typical 52% phosphoric acid production, a 60 torr vacuum is applied in a boiler and 85° C. phosphoric acid is circulating and heated up by a heat exchanger at 130° C. During this evaporation process, some scale is formed either at the boiler or at the heat exchanger. However, the scale formed at the boiler can be different than that formed at the heat exchanger. The slow forming scale such as that formed at the heat exchanger, contain mostly magnesium fluoro-aluminates; while the fast forming scale such as that formed on pipes, contain mostly sodium or potassium fluorosilicate.
Yet, there is very little information in addressing the phosphoric acid plant scale issue in an industrial setting. Even for academic studies, the results are scattered. For example, several articles mention reagents for fluorosilicate inhibition in phosphoric acid production. (See L. Yang, Zhang Y., Huang, Y. Chemical Industry and Engineering (China), (2002), V 19(1), 1). Chinese patent CN1762857 discloses that mixtures of phosphonic acids, polymers, and pesticides can effectively reduce the scale formation in wet-process phosphoric acid production. U.S. Pat. No. 5,120,519 discloses that high molecular weight polyacrylamide and polyacrylic acid can prevent scale from adhering on the surface of the phosphate rock and phosphoric acid. However, the use of most of these chemicals is not new and has been applied in the water treatment system for scale control, and the mechanism of these reagents is based mostly on their dispersant effect.
Accordingly, the compositions and methods presently available for preventing and/or reducing scale in the phosphoric acid production process require further improvement. Compositions and formulations that effectively prevent and/or reduce scale, thereby enabling the phosphoric acid production plant to run longer without shutting down to remove scale would be a useful advance in the art and could find rapid acceptance in the industry.